The most important category is support and billing terms. Handles like support, help, billing, payments, and account are used in nearly every phishing attack. When @support messages a user asking for their password, many will comply. These handles need to be reserved or controlled by your actual support team.
Second priority is security-related terms. Reserve security, secure, verify, verified, confirm, auth, login, and similar. These appear in phishing URLs and credential theft schemes. An attacker with the @verify handle can send convincing "verify your account" messages.
Third is your brand and official accounts. Your company name, product names, and official designations (like company_official or company_support) must be locked down. Attackers register these handles on new platforms specifically to impersonate companies.
Finally, administrative terms like admin, moderator, staff, and team. These suggest authority. Users trust messages from accounts with these names. Reserve them even if you do not plan to use them — it is about preventing misuse, not your own usage.